In a typical scenario (e.g. in a specific geographical area) the available radio communication spectrum is sub-divided into spectrum parts that are dedicated to communication and/or other activities subject to restrictions from a frequency allocation body (e.g. the International Telecommunication Union—ITU) and spectrum parts that are dedicated to communication and/or other activities not subject to such restrictions.
Example activities subject to these restrictions include cellular telecommunication (e.g. according to the Global System for Mobile communication (GSM) standard, the Universal Mobile Telecommunication Standard (UMTS), the UMTS Long Term Evolution (UMTS LTE) standard, or the UMTS LTE-Advanced (UMTS LTE-A) standard), radio astronomy, television, air traffic control, etc.
The spectrum parts that are not subject to these types of restrictions will be referred to herein as unlicensed frequency bands. The spectrum parts that are only partly subject to these types of restrictions will be referred to herein as non-exclusively licensed frequency bands. Example activities in unlicensed frequency bands include Wireless Local Area Network (WLAN) communication (e.g. based on the IEEE—Institute of Electrical and Electronics Engineers—802.11 standard), Bluetooth communication, cordless phone signaling, remote control signaling, etc. The Industrial, Scientific and Medical (ISM) radio bands are examples of unlicensed frequency bands.
Spectrum sharing for WLAN is typically achieved by dividing the total available bandwidth into a number of channels. The channels are typically partially overlapping and communication in adjacent channels will, thus, interfere with each other.
In the 2.4 GHz band, the channels are typically 20 MHz wide and up to 13 channels are defined. In a typical utilization, three non-overlapping channels are used in the 2.4 GHz band to avoid adjacent channel interference.
In the 5 GHz band, many more channels are available since the available bandwidth is much larger. However, with the development of IEEE 802.11n and IEEE 802.11ac, the potential bandwidth of each channel has been increased from 20 MHz to 40, 80, and even 160 MHz. Thus, the number of non-overlapping channels available may still be rather small, in particular when the wider channel bandwidths are used.
FIG. 1 schematically illustrates an available bandwidth 100 divided into eight channels 101-108 (e.g. of 20 MHz each). Depending on the particular choice of IEEE 802.11 application, channels of 20 MHz (e.g. 110, 115), 40 MHz (e.g. 120, 125), 80 MHz (130, 135) or 160 MHz (not shown) may be used.
In a typical WLAN deployment, the channel allocation to the access points may be such that the channels used by neighboring access points, as far as possible, are not overlapping. In practice, this is often achieved by aiming to maximize the distance between access points that use the same (and/or adjacent) channel(s).
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is typically used in WLAN systems to further lower the interference between WLAN devices. When CSMA/CA is employed, the channel to be used is first sensed (typically by performing suitable measurements), and a transmission is only initiated if the channel is declared free (i.e. un-used, also termed Idle). If the channel is declared occupied (i.e. already used, also termed Busy), the intended transmission is typically deferred until the channel is found to be Idle. The CSMA/CA approach may lead to time-sharing of a channel between two or more access points which are in range of one another and using the same or overlapping channels, which may lower the throughput of one or more of the access points.
Some WLAN systems (e.g. IEEE 802.11n and IEEE 802.11ac) define a structure where each channel is defined as a primary or secondary channel by the access point. The information regarding which channels are primary and secondary channels is broadcast by the access point in a beacon signal (which is typically sent periodically). In such a system, a WLAN station (STA) has to sense the primary channel of the access point and declare it to be Idle before it may access the channel, and the same applies to the secondary channel. However, if the secondary channel is Idle and the primary channel is Busy, the secondary channel still cannot be used. In the example of FIG. 1, channels 110, 125 and 130 may be primary channels, and channels 115, 120 and 135 may be respective secondary channels. Typically, the access point determines which channels are to be used as primary and secondary channels, respectively.
The unlicensed frequency bands are widely used by WLAN communication (e.g. 802.11n and 802.11ac). However, there are typically still a lot of available frequency resources in the unlicensed bands. It might be desirable to use such available frequency resources in the unlicensed frequency bands for cellular telecommunication (e.g. UMTS LTE). Introducing UMTS LTE-A operation in unlicensed bands would allow for more flexible use of the unlicensed bands, especially if WLAN access points (AP) cooperate with the network nodes of the cellular communication system.
Introduction of UMTS LTE-A communication in unlicensed bands may, for example, comprise using a licensed frequency band for a primary cell (PCell) of a carrier aggregation scheme and using the unlicensed frequency band for a secondary cell (SCell) of the carrier aggregation scheme. In this manner, a connection between the UMTS LTE-A network and a wireless communication device is still maintained via the PCell if the SCell is severely interfered (e.g. by WLAN or Bluetooth). Examples of carrier aggregation using unlicensed bands are disclosed in US 2013/0195073 A1 and US 2014/0044105 A1.
FIG. 2 illustrates an example scenario where a cellular communication 215 between a wireless communication device 210 and a network node 200 is taking place in the vicinity of two access points 220, 230 for WLAN communication. The WLAN communication uses unlicensed frequency resources, and may include the access points 220, 230 transmitting respective beacon signals 225, 235. The cellular communication 215 typically uses one or more licensed frequency bands, but may (alternatively or additionally) also use unlicensed frequency resources.
WLAN standards are primarily designed so that WLAN devices may smoothly coexist with other WLAN devices. Therefore, if UMTS LTE-A is employed in an unlicensed frequency band already used by WLAN, the performance for the WLAN system may be severely degraded. On the other hand, employing WLAN-type approaches (e.g. CSMA/CA) may be extraneous to, and inflict very strong restrictions on, UMTS LTE-A.
Therefore, there is a need for alternative approaches of deployment of UMTS LTE-A in an unlicensed or non-exclusively licensed band which may (potentially) be used by WLAN. Preferably, the approaches should enable high data rates in UMTS LTE-A while ensuring minimum (or at least acceptable) performance loss for WLAN.